In a fluorometric assay there is directed to the sample a short-wave excitation light, which makes the substance being assayed to emit longer-wave light. The quantity of the emitted light is measured, and thereby the quantity of the substance being assayed is detected. In general, a fluorometer has one measuring channel and the assays are performed on plates having multiple wells. In this case the plate is moved relative to the measuring channel in such a way that each well in turn arrives at the measuring position.
The background fluorescence caused by excitation light incident outside the sample being assayed constitutes one problem with fluorometers. This can be reduced by using non-transparent shields. In practice, however, it is difficult to obtain sufficient light-tightness in this manner.
Fluorometric measuring through a vessel is not especially recommendable, since in such a case there passes through the vessel also a large amount of excitation light, which complicates the measuring of emitted light. Background fluorescence possibly caused by the material of the vessel constitutes a further problem. In most fluorometers currently used for routine assays, excitation light is directed to the vessel from above, and also emitted light is collected from above. In certain assays it is, nevertheless, best to carry out the measuring through the bottom of the sample-holding vessel.
From publication EP-A-108524 there is known a fluorometer measuring from above. Therein the excitation light is delimited by means of an aperture, a diverging beam of light is converged by means of a lens, and the converging beam of light is directed via a filter and a mirror as a spot to the sample-holding vessel. The emitted light, respectively, is directed from the vessel via another mirror, filter, converging lens and delimiting aperture to a detector.
From publication EP-A-640828 there is known a fluorometer measuring from above, wherein excitation light is directed simultaneously to a plurality of samples through a dichroic mirror, and emitted light from the sample vessels is reflected via the same mirror to a camera.